A liquid crystal display device includes a TFT substrate on which pixel electrodes and thin-film transistors (TFTs) are formed in a matrix and a substrate that is opposed to the TFT substrate and has a color filer or the like located so as to face the pixel electrodes of the TFT substrate. A liquid crystal is interposed between the TFT substrate and the opposed substrate. The light transmittance of liquid crystal molecules is controlled for each pixel so as to form an image.
Liquid crystal display devices are flat and lightweight and thus have been widely used in various fields, for example, large display devices such as a television, cellular phones, and digital still cameras (DSCs). Moreover, liquid crystal display devices have disadvantageous viewing angles. In other words, an image is viewed differently depending on a viewing angle with respect to a screen. Regarding viewing angles, liquid crystal display devices of in plain switching (IPS) have excellent characteristics.
In a liquid crystal display device, video signal lines are extended in a first direction and arranged in a second direction while scanning lines are extended in the second direction and arranged in the first direction. An Al alloy is used for the video signal line to reduce the line width and electric resistance. An Al alloy is likely to become defective during etching or development in a photolithographic process.
Various IPS liquid crystal display devices are available. In some methods, video signal lines or drain electrodes and pixel electrodes of indium tin oxide (ITO) are formed on the same layer (e.g., a gate insulating film), an interlayer insulating film is formed thereon, and comb-shaped counter electrodes are formed thereon. In this case, unfortunately, a developer of ITO dissolves the video signal lines or the drain electrodes, leading to defects or breaks.
In order to solve the problem, Japanese Patent Laid-Open No. 2012-73341 describe a configuration in which ITO pixel electrodes are first formed and then the drain electrodes of TFTs or video signal lines are formed. For the same purpose, Japanese Patent Laid-Open No. 2011-145530 describes a configuration in which ITO contained in pixel electrodes forms a two-layer structure for preventing a developer of ITO from damaging drain electrodes or video signal lines.